fix(plugin-giscus): fix giscus lang options setting

[Zhengqbbb]

Related ISSUE

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issue: #57

Type Of Change

• [x] 🐞 Bug fix (non-breaking change which fixes an issue)
• [ ] ✨ New feature (non-breaking change which adds functionality)
• [ ] 📝 Document (This change requires a documentation update)
• [ ] 🎨 Theme style (Theme style beautification)
• [ ] ⚠ Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] 🔨 Workflow (Workflow changes)

Clear Describe

A clear and concise description of what update for target.

• fix(plugin-giscus): fix giscus lang to support page lang
• fix(plugin-giscus): fix overwrite giscus lang options of theme config

Description

1. The current language setting only supports the user to set the locales lang attribute. If the user does not set it, in the case of a single language, the lang attribute of the web page should be obtained and set.
2. Should support user overwrite configuration items

Test Case

Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce. Please also list any relevant details for your test case.

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